1. Field of the Invention
The present invention relates to a display in which the angular output range of light is controllable, so that the display can be switched between a wide angle viewing mode and a narrow angle viewing mode.
2. Description of the Related Art
Electronic display devices such as, for example, monitors used with computers and screens built in to mobile telephones and other portable information devices, are usually designed to have as wide a viewing angle as possible, so that an image displayed by the device can be seen from many different viewing positions. However, there are some situations where it is desirable for an image displayed by a device to be visible from only a narrow range of viewing angles. For example, a person using a portable computer in a crowded train might want the display screen of the computer to have a small viewing angle so that a document displayed on the computer screen cannot be read by other passengers on the train. For this reason, there has been considerable effort put in to developing display devices which are electrically switchable between two modes of operation—in a ‘public’ display mode they have a wide viewing angle for general use, but they can be switched to a ‘private’ display mode in which they have a narrow viewing angle so that private information can be displayed in public places without being visible to people other than the user of the device.
Another application of such a display may be as a display in a motor vehicle. The viewing angle of the display could be controlled such that the passengers are unable to see the display or such that the driver is unable to see the display. Alternatively the viewing angle could be controlled in order to reduce the reflections of the display on the windshield and the windows so that, for example, the viewing angle could be reduced at nighttime or in low light conditions. A brightness sensor could be provided to allow automatic switching between a wide viewing angle and a narrow viewing angle, and also to allow automatic control of the brightness of the display.
A number of devices are known which restrict the range of angles or positions from which a display can be viewed.
U.S. Pat. No. 6,552,850 describes a method for the display of private information on an automatic teller machine (ATM). Light emitted by the machine's display has a fixed polarization state, and the machine and its user are surrounded by a large screen of sheet polarizer which absorbs light of that polarization state but transmits light of a perpendicular polarization state. Passers-by can see the user and the machine, but cannot see information displayed on the machine's screen.
One known element for controlling the direction of light is a ‘louvered’ film that consists of alternating transparent layers and opaque layers provided in an arrangement similar to a Venetian blind. The film operates on the same principle as a Venetian blind, and it allows light to pass through it when the light is traveling in a direction parallel to, or nearly parallel to, the opaque layers. However, light traveling at large angles relative to the plane of the opaque layers is incident on one of the opaque layers and is absorbed. The layers may be perpendicular to the surface of the film or they may be at some other angle relative to the surface of the film.
Louvered films of this type may be manufactured by stacking many alternating sheets of transparent material and opaque material and then cutting slices of the resulting block perpendicular to the layers. This method has been known for many years and is described in, for example, U.S. Pat. Nos. 2,053,173; 2,689,387 and 3,031,351.
Other manufacturing methods are known. For example, U.S. Pat. No. RE 27,617 describes a process where a louvered film is cut continuously from a cylindrical billet of stacked layers. U.S. Pat. No. 4,766,023 describes how the optical quality and mechanical robustness of the resulting film can be improved by coating with a UV-curable monomer and then exposing the film to UV radiation. U.S. Pat. No. 4,764,410 describes a similar process where a UV-curable material is used to bond the louver sheet to a covering film.
Other methods exist for making films with similar properties to the louvered film. For example, U.S. Pat. No. 5,147,716 describes a light-control film which contains many elongated particles which are aligned in the direction perpendicular to the plane of the film. Light rays which make large angles to this direction are therefore strongly absorbed, whereas light rays propagating in this direction are transmitted.
Another example of a light-control film is described in U.S. Pat. No. 5,528,319. This film has a transparent body in which are embedded opaque regions that extend generally parallel to the plane of the film. The opaque regions are arranged in stacks, with each stack being spaced from a neighboring stack. The opaque regions block the transmission of light through the film in certain directions while allowing the transmission of light in other directions.
The prior art light control films may be placed either in front of a display panel or between a transmissive display panel and its backlight, to restrict the range of angles from which the display can be viewed. In other words, the prior art light control films make a display ‘private’. However, none of the prior art light control films enables the privacy function to be switched off to allow viewing from a wide range of angles.
There have been reports of a display which can be switched between a public mode (with a wide viewing angle) and a private mode (with a narrow viewing angle). For example, US Patent Application Publication No. 2002/0158967 suggests that a light control film could be movably mounted on a display so that the light control film either may be positioned over the front of the display to provide a private mode or may be mechanically retracted into a holder behind or beside the display to give a public mode. This method has the disadvantage that it contains moving parts which may fail or be damaged in use, and which add bulk to the display.
A method for switching a display panel from public to private mode with no moving parts is to mount a light control film behind the display panel, and to place a diffuser which can be electronically switched on and off between the light control film and the panel. When the diffuser is inactive, the light control film restricts the range of viewing angles and the display is in a private mode. When the diffuser is switched on, the light with a narrow angle range output from the light control film is incident on the diffuser, and the diffuser acts to increase the angular spread of the light. That is, the diffuser cancels out the effect of the light control film. Thus, the display is illuminated by light traveling at a wide range of angles and the display operates in a public mode. It is also possible to mount the light control film in front of the panel and place the switchable diffuser in front of the light control film to achieve the same effect.
Switchable privacy devices of the above type are described in U.S. Pat. Nos. 5,831,698; 6,211,930; and 5,877,829. They have the disadvantage that the light control film always absorbs a significant fraction of the light incident upon it, whether the display is in public mode or private mode. The display is therefore inherently inefficient in its use of light. Furthermore, since the diffuser spreads light through a wide range of angles in the public mode, these displays are also dimmer in public mode than in private mode (unless the backlight is made brighter when the device is operating in public mode to compensate).
Another disadvantage of these devices relates to their power consumption. Such devices often use a switchable polymer-dispersed liquid crystal diffuser which is not diffusive when no voltage is applied across the liquid crystal layer and which is switched on (into the diffusive state) by applying a voltage. Thus, to obtain the public mode of operation it is necessary to apply a voltage across the diffuser so that the diffuser is switched on. More electrical power is therefore consumed in the public mode than in the private mode. This is a disadvantage for mobile devices which are used for most of the time in the public mode and which have limited battery power.
Another method for making a switchable public/private display is described in U.S. Pat. No. 5,825,436. The light control device in this patent is similar in structure to the louvered film described above. However, each opaque element in a conventional louvered film is replaced by a liquid crystal cell which can be electronically switched from an opaque state to a transparent state. The light control device is placed in front of or behind a display panel. When the cells are opaque, the display operates in a private mode; when the cells are transparent, the display operates in a public mode.
One significant disadvantage of this device is the difficulty and expense of manufacturing liquid crystal cells with an appropriate shape. A second disadvantage is that, in the private mode, a ray of light may enter at an angle such that it passes first through the transparent material and then through part of a liquid crystal cell. Such a ray will not be completely absorbed by the liquid crystal cell and this may reduce the privacy of the device.
Japanese Patent Application No. 2003-28263 describes a switchable viewing angle control mechanism for a liquid crystal (LC) panel. This uses an additional twisted nematic (TN) LC panel, which is patterned in a checkerboard pattern. In the narrow viewing mode, the limited viewing angle characteristics of a standard TN LC panel are used to make a checkerboard pattern appear when the LC panel is viewed from an angle well away from the normal direction. This checkerboard pattern is confusing for the viewer and degrades the quality of the image seen from an angle well away from the normal direction. This does have the disadvantage that an additional LC panel and an additional polarizer are required.
Japanese Patent Application No. 9-105958 describes a display having a switchable view angle control device, containing a liquid crystal material, placed in the path of light from an image display device. In a narrow display mode, the liquid crystal molecules of the view angle control device are oriented vertically (that is, perpendicular to the substrates and parallel to the normal axis of the display).
While the display of Japanese Patent Application No. 9-105958 can provide a narrow display mode, it is difficult to switch the view angle control device to give a wide display mode. One way of obtaining a wide display mode is to switch the liquid crystal material of the view angle control device to an isotropic state. However, this requires heating the liquid crystal material to a temperature at which it becomes isotropic, and this is undesirable. Alternatively, a wide display mode may be obtained by switching the liquid crystal material of the view angle control device such that the liquid crystal molecules are aligned horizontally (that is, parallel to the substrates). However, this requires switching the liquid crystal molecules between a vertical alignment and a horizontal alignment, and this is very hard to do in practice.
Y. Hisatake et al. disclose, in SID Digest '05, page 1218, a display having a switchable view angle control device, containing a liquid crystal material, placed in the path of light from an image display device. The view angle control device is an ultra super twisted (“UST”) liquid crystal cell, that is switchable between a positive retardation state and a negative retardation state. The image display device is a twisted nematic liquid crystal cell. When the UST liquid crystal cell has a positive retardation, the viewing angle of the display is less than the viewing angle of the TN liquid crystal cell, whereas when the UST liquid crystal cell has a negative retardation, the viewing angle of the display is greater than the viewing angle of the TN liquid crystal cell.
The “Rocket” software provides a further method of switching a display between a public mode and a private mode. This software makes use of the fact that a grey level curve of a liquid crystal display is angle dependent, and may show contrast inversion at some viewing angles. It obtains a private viewing mode by operating the device in a voltage range which produces contrast inversion as high viewing angles, so that an image can be seen only at viewing angles close to the normal direction of the display. This approach has the disadvantage that it cannot obtain a good private viewing mode with newer liquid crystal materials that have grey level curves with a much reduced dependence on viewing angle.